CODEWITHSUNDEEP
exceptionarmundefined
sobin thomas
sobin thomas

Reputation: 59

What is the cause of undefined ARM Exceptions?

One question is when the undefined instruction happens .... Do we need to get the current executing instruction from R14_SVC or R14_UNDEF? . Currently I am working on one problem where an undefined instruction happened. On checking the R14_SVC I found the instruction was like below:

0x46BFD73C cmp r0, #0x0
0x46BFD740 beq 0x46BFD75C
0x46BFD744 ldr r0,0x46BFE358

so in my assumption the undefined instruction would have happened while executing the instruction beq 0x46BFD75C

One thing that puzzles me is I checked the r14_undef and the istruction was different.

0x46bfd4b8  bx r14
0x46bfd4bC  mov r0, 0x01
0x46bfd4c0  bx r14

Which one caused the undefined instruction exception?

Upvotes: 2

Views: 4446

Answers (1)

old_timer
old_timer

Reputation: 71506

All of your answers are in the ARM ARM, ARM Architectural Reference Manual. go to infocenter.arm.com under reference manuals find the architecture family you are interested in. The non-cortex-m series all handle these exceptions the same way

When an Undefined Instruction exception occurs, the following actions are performed:
R14_und = address of next instruction after the Undefined instruction
SPSR_und = CPSR
CPSR[4:0] = 0b11011 /* Enter Undefined Instruction mode */
CPSR[5] = 0 /* Execute in ARM state */
            /* CPSR[6] is unchanged */
CPSR[7] = 1 /* Disable normal interrupts */
            /* CPSR[8] is unchanged */
CPSR[9] = CP15_reg1_EEbit
/* Endianness on exception entry */
if high vectors configured then
    PC = 0xFFFF0004
else
    PC = 0x00000004

R14_und points at the next instruction AFTER the undefined instruction. you have to examine SPSR_und to determine what mode the processor was in (arm or thumb) to know if you need to subtract 2 or 4 from R14_und and if you need to fetch 2 or 4 bytes. Unfortunately if on a newer architecture that supports thumb2 you may have to fetch 4 bytes even in thumb mode and try to figure out what happened. being variable word length it is very possible to be in a situation where it is impossible to determine what happened. If you are not using thumb2 instructions then it is deterministic.

Upvotes: 3

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